The use of herbicides for weed control is an extremely valuable agricultural practice to protect the yield of crop plants and to manage the growth of vegetation in pastures and other sites. However the use of many herbicides is becoming problematic through the advent of field resistance and also from increased toxicological and environmental concerns associated with certain pesticidal chemistries and modes of action. Thus there is a continuing need for new herbicidal chemistries.
There are currently a limited number of herbicidal classes that have proven to exhibit the desired attributes for a beneficial modern herbicide such as low incidence of resistance development and low potential for toxicological side effects. One such class is the auxinic herbicides that include such compounds as 2,4-D and picloram. This class can be further subdivided into compounds that contain a picolinate moiety (e.g., picloram, aminopyralid, clopyralid), those that contain an aryloxyacetate moiety (e.g., 2,4-D, fluoroxypyr, triclopyr etc.) and others (e.g., dicamba). All of these compounds elicit plant symptoms similar to excessive treatment with the natural plant hormone indole acetic acid (IAA) and induce similar physiological events, eventually leading to plant death. Interestingly, the widespread use of auxinic herbicides for many years has not resulted in significant field resistance. In addition, the auxinic mode of action is specific to plants and many of the auxinic herbicides exhibit favorable environmental profiles. Thus methods for the discovery of new compounds that act via the auxinic mode of action would be of great benefit, particularly those with increased potency, wider spectrum, optimal soil persistency or low cost of manufacture.
Genes that confer resistance to certain herbicides have found utility in the development of herbicide-tolerant crops by transgenic or mutagenic selection methods. Many auxinic herbicides have broad spectrum herbicidal activity but their use is limited by the fact that the desired crop or pasture component species is sensitive to the herbicide. This is particularly the case for the picolinate class of auxinic herbicides. Thus there is an unfulfilled need for herbicide-tolerance mechanisms that would be applicable to the picolinate class of auxinic herbicides.